Thermally crystallizable glasses and glass-ceramics made therefrom

ABSTRACT

Thermally crystallizable glass and glass-ceramic compositions, having a narrow critical range of compositions coming within the SiO 2  -Al 2  O 3  -Li 2  O system, consist essentially of SiO 2 , Al 2  O 3 , Li 2  O, TiO 2  and ZrO 2 , contain 18.2-20.3 parts Al 2  O 3  per 100 parts by weight of SiO 2  +Al 2  O 3  +Li 2  O, have Li 2  O to Al 2  O 3  molar ratio of 0.55 to 0.63, and contain 2.8-5 weight percent TiO 2  +ZrO 2 . Glass-ceramics of superior static fatigue resistance can be made from such glasses.

The present invention relates to thermally crystallizable glasses and glass-ceramics having a narrow critical range of compositions coming within the SiO₂ -Al.sub. 2 O₃ -Li₂ O system.

In U.S. Pat. No. 3,809,599 is described a glass-ceramic inner liner for a gas turbine engine and a method of making such an inner liner. The patent also discloses a number of glass-ceramic compositions having certain superior properties for use as an inner liner in such a gas turbine.

Experience with gas turbine inner liners has revealed a need for a glass-ceramic having the properties disclosed in said U.S. Pat. No. 3,809,599 and in U.S. Pat. No. 3,970,463, which describes such compositions in more detail, but also having improved static fatigue resistance when compared to compositions disclosed in U.S. Pat. No. 3,970,463.

It has now been found that certain glass compositions in the SiO₂ -Al.sub. 2 O₃ -Li₂ O field can be thermally crystallized to excellent low expansion glass-ceramic materials and that a critical range of such compositions can be crystallized, with proper heat treatment, to glass-ceramics materials having improved static fatigue resistance at high temperatures and improved resistance to permanent deflection. Moreover, such glasses can be crystallized to glass-ceramics having a low average linear coefficient of thermal expansion (α) of less than 15×10⁻⁷ /° C over the range from 25° to 700° C, as well as a flexural strength at 1700° F of at least 12,000 psi after grinding or abrading the surface, and excellent dimensional stability under conditions which will be described hereafter.

The glass and glass-ceramic compositions of the present invention consist essentially of SiO₂, Al.sub. 2 O₃, Li₂ O, and nucleants TiO₂ and ZrO₂. When felt necessary my compositions can contain as fining agents small amounts of antimony oxide and/or arsenic oxide, which are preferably present in no more than 0.5 weight percent total, where the oxides are expressed as Sb₂ O₃ and As₂ O₃, respectively. Sb₂ O₃ is now preferred.

My compositions, when expressed on the basis of 100 weight parts SiO₂ +Al.sub. 2 O₃ +Li₂ O, contain 18.2-20.3 weight parts Al.sub. 2 O₃. The glasses and glass-ceramics of the invention contain 0.55 to 0.63 moles of Li₂ O for each mole of Al.sub. 2 O₃ and contain 2.8 to 5 weight percent TiO₂ +ZrO₂, based on the total composition, including 1- 2 percent TiO₂ and 1.2-3 percent ZrO₂. Usually TiO₂ is in the range of 1.2-1.9 weight percent and ZrO₂ in the range from 1.4-2 weight percent.

I have discovered in particular that the lower molar Li₂ O/Al.sub. 2 O₃ ratio of my compositions (which contain the restricted 18.2-20.3 weight percent range of Al.sub. 2 O₃ noted above) as compared to the compositions in U.S. Pat. No. 3,970,463, makes it possible to prepare glass-ceramics having much better fatigue resistance than when using the Li₂ O/Al.sub. 2 O₃ ratios recommended in said patent.

It is important that the compositions of the invention, aside from fining agents, consist essentially solely of the three essential ingredients plus the nucleating agents coming within the weight amounts and molar ratios defined above. It is imperative that the impurities be kept to an absolute minimum. While no impurities should be present it is recognized that it is almost impossible to make glass without some impurities unless only pure materials were used in the batch. This is not possible on any commercial scale because of the costs. However, care should be maintained in selecting batch materials for making the glasses to insure that impurities in the resulting glasses are kept as low as practicable.

Thus, Na₂ O and K₂ O are usually held to no more than 0.2 weight percent total and preferably no more than 0.1 weight percent. Small amounts of fluoride ion are very detrimental to static fatigue resistance and F should be held to less than 0.1 weight percent or altogether eliminated. Iron oxide when present as an impurity can be tolerated in an amount less than 0.5 weight percent, expressed as Fe₂ O₃. ZnO tends to lower the coefficient of thermal expansion, which is usually to be desired, but it decreases the static fatigue resistance and the rate of permanent deflection or deformation so, if used at all, should be present in amounts of 0.2 weight percent of the composition, or less.

In a specific example of the invention a glass batch was melted using the following batch materials in the indicated amounts:

    ______________________________________                                                        Parts by Weight                                                 ______________________________________                                         Ottawa silica.sup.1                                                                             10569.4739                                                    A-10 alumina.sup.2                                                                              2788.9731                                                     Lithium carbonate.sup.3                                                                         822.7151                                                      Antimony trioxide.sup.4                                                                         44.9995                                                       ZrO.sub.2 frit.sup.5                                                                            1045.2856                                                     Lithium nitrate.sup.6                                                                           157.5000                                                      Titanox (TiO.sub.2).sup.7                                                                       217.4282                                                      (Fusion loss     646.3752)                                                     ______________________________________                                          .sup.1 99.54 wt% SiO.sub.2, 0.0130 wt% TiO.sub.2, 0.0500 Al.sub.2 O.sub.3      0.0190 Fe.sub.2 O.sub.3                                                        .sup.2 99.67 wt% Al.sub.2 O.sub.3, 0.12 SiO.sub.2, 0.032 Fe.sub.2 O.sub.3      0.13 Na.sub.2 O                                                                .sup.3 40.0000 wt% Li.sub.2 O, 0.0005 wt% Fe.sub.2 O.sub.3, balance            CO.sub.2                                                                       .sup.4 100% Sb.sub.2 O.sub.3                                                   .sup.5 65.4000 wt% SiO.sub.2, 22.9600 ZrO.sub.2, 0.0029 Fe.sub.2 O.sub.3,      11.9000 Li.sub.2 O                                                             .sup.6 100% LiNO.sub.3                                                         .sup.7 99.4000 wt% TiO.sub.2, 0.0113 Fe.sub.2 O.sub.3                    

This batch was thoroughly mixed and then melted in a gas-fired furnace with mechanical stirring at a temperature of 3100° F for about 19 hours, after which the temperature was lowered to 2850° F and cane was drawn. The melt was then reheated to 3100° F and one inch thick slabs were cast. The cane and the slab samples were of good quality. They were annealed at 1400° F for 2 hours.

The glass so made was the glass of composition 2 shown in Table I. In addition to the components shown in Table I composition 2 contained as impurities Na₂ O and iron oxide expressed as Fe₂ O₃ in the amounts of less than 0.025 and less than 0.02 weight percent Na₂ O and Fe₂ O₃, respectively.

The glass samples were heat treated for crystallization to a glass-ceramic by heating according to the following schedule:

    ______________________________________                                         Heating or                                                                     Cooling                    Hours at                                            Rate, ° F/Hr.                                                                        Temp. ° F                                                                             Temperature                                         ______________________________________                                         100          1350          0                                                   10           1400          0                                                   5            1700          0                                                   10           1900          0                                                   25           2200          12                                                  10           1800          0                                                   (Furnace rate to room temperature)                                             ______________________________________                                    

It should also be noted that compositions 1 and 3-5 in Table I were also heat treated according to the same schedule.

The glass-ceramic so produced was tested for static fatigue resistance, permanent deflection under load, linear coefficient of thermal expansion, dimensional stability and flexural strength in the manner described hereafter. Results are shown in Table I, as well as test results for compositions 1 and 3-10, which were melted and formed as glasses and heat treated in a manner similar to that described with respect to composition 2. However, the heat treatment schedule for compositions 6-10 of Table I was somewhat different and was as follows:

    ______________________________________                                         Heating or                                                                     Cooling                    Hours at                                            Rate, ° F/Hr.                                                                        Temp. ° F                                                                             Temperature                                         ______________________________________                                         100          1350          0                                                   10           1400          0                                                   5            2050          0                                                   25           2200          12                                                  10           1800          0                                                   (Furnace rate to room temperature)                                             ______________________________________                                    

Either of the foregoing heat treatment schedules are satisfactory to crystallize any glass of the invention to a glass-ceramic of the invention.

As will be understood the lower initial stages of heat treatment involve nucleation and the later stages involve growth of small crystals, according to known principles. Details of methods of determining optimum nucleation temperature ranges and crystallization temperature for crystallizable SiO₂ ·Al.sub. 2 O₃ ·Li₂ O glasses are set forth in U.S. Pat. Nos. 3,625,718; 3,615,759; and 3,380,818. Many other heat treatment schedules will produce glass-ceramics from the presently claimed glasses. I have found in general, however, that the high temperature crystallizing portion of the heat treatment should be over 2100° F, usually at least about 2200° F to obtain glass-ceramics having the most improved static fatigue resistance. Shorter times of heating, at the lower temperatures particularly, can be used than in the schedules shown, if the article to be crystallized is relatively thin. The heat treatment schedules shown are such that they can be used successfully on large castings.

The static fatigue test was effected at 1700° C. Centerless ground glass-ceramic rods of about 0.2 inches in diameter were supported on two alumina knife edges spaced 2 inches apart. The grinding was accomplished with a 320 grit SiC wheel. A constant load was applied at the center of the rods, and the load was such as to result in a stress of 12,000 psi according to the well known formula, ##EQU1## where the Load is in pounds and the diameter in inches. For a 0.2 inch diameter rod the load was 18.8 pounds; for rods whose diameters deviated slightly from the 0.2 inch, standard, the load was varied to make the stress equal the 12,000 psi standard. If a rod broke before 200 hours under such load at 1700° F, it was considered to have failed.

The permanent deflection was determined on rods prepared, supported and loaded in exactly the same manner as described with respect to the static fatigue test, and this test was carried out at the same 1700° F temperature. If a given rod reached 200 hours without breaking, the amount of vertical mid-point deflection in inches was measured at this time and usually also later if the sample survived for a longer time. If the sample broke before 200 hours it was considered not to be satisfactory simply because of the breakage under fatigue before this interval of time. Compositions of the invention can be thermally crystallized to glass-ceramics having a mid-point deflection in such test of less than 0.01 inches after 200 hours at 1700° F test temperature.

In Table I is shown both the average rate of deflection per hour and the total deflection after 200 hours if the sample survived 200 hours.

The tested flexural strength (modulus of rupture) of the crystallized material was determined using crystallized cane samples, of about 0.20 inch in diameter. The modulus of rupture tests were made using a Dillon Universal Tester. This machine applies a measured load through a single knife edge to the center of a 2 inch long sample of cane supported on two knife edges (3-point loading). The load is applied at a constant rate of about 8.2 pounds per minute until failure occurs. The test was run at the temperature indicated in the table, either room temperature or 1700 ° F. The modulus of rupture is calculated according to the formula: ##EQU2## where Load and D are as before defined. The rods were ground in the same manner as described with respect to the static fatigue test. However, rods ground before crystallization and rods ground after crystallization were both tested. Results are in pounds per square inch. It should be noted that the rate of loading used in this test was slower than usual, leading to conservative (i.e., lower) values for the breaking strength.

The dimensional stability was determined by heating crystallized rods for 1000 hours at 1800° F and then measuring the change in length of the rod in parts per million. The length measurements were made before heating and after heating, both measurements being at room temperature. Values are reported in parts per million. All compositions of the invention can be crystallized to glass-ceramics that undergo a change in dimension of less than 300 ppm in this test.

In Table I the compositions are given in weight percent. In Table II the weight percentages of SiO₂, Al₂ O₃ and Li₂ O are shown on the basis of 100 parts of SiO₂, Al₂ O₃ and Li₂ O.

                                      TABLE I                                      __________________________________________________________________________               Composition:                                                                           1    2    3   4   5   6    7    8    9   10                  __________________________________________________________________________                Component                                                                     SiO.sub.2                                                                              74.79                                                                               74.79                                                                               74.02                                                                              74.71                                                                              76.76                                                                              74.79                                                                               74.79                                                                               75.76                                                                               74.53                                                                              74.02                         Al.sub.2 O.sub.3                                                                       18.11                                                                               18.61                                                                               17.92                                                                              18.07                                                                              16.44                                                                              18.42                                                                               18.72                                                                               17.74                                                                               18.55                                                                              18.42                         Li.sub.2 O                                                                             3.75 3.25 3.71                                                                               3.74                                                                               3.40                                                                               3.40 3.10 3.11 3.24                                                                               3.21                          TiO.sub.2                                                                              1.45 1.45 1.45                                                                               1.45                                                                               1.45                                                                               1.45 1.45 1.45 1.45                                                                               1.45                          TiO.sub.2                                                                              1.45 1.45 1.45                                                                               1.45                                                                               1.45                                                                               1.45 1.45 1.45 1.45                                                                               1.45                          ZrO.sub.2                                                                              1.60 1.60 2.60                                                                               1.60                                                                               1.60                                                                               1.60 1.60 1.60 1.60                                                                               1.60                          Sb.sub.2 O.sub.3                                                                       0.30 0.30 0.30                                                                               0.30                                                                               0.30                                                                               0.30 0.30 0.30 0.30                                                                               0.30                          F.sub.2 --   --   --  0.20                                                                               --  --   --   --   --  --                            ZnO     --   --   --  --  --  --   --   --   0.33                                                                               1.00                 Li.sub.2 /Al.sub.2 O Mol Ratio                                                                  0.707                                                                               0.596                                                                               0.706                                                                              0.706                                                                              0.706                                                                              0.629                                                                                0.564                                                                              0.597                                                                               0.596                                                                              0.595                 α(25-700° C)×10.sup.7                                                       0.0  9.8  --  --  --  7.8  10.1 9.8  5.7 6.7                 Deflection, Rate, inches/Hr×10.sup.4                                                       0.918                                                                               0.193                                                                               --  --  --  0.2975                                                                              0.1725                                                                              0.2609                                                                              0.66                                                                               1.45                 Total inches/200 Hrs×10.sup.4                                                             F.sup.1                                                                             40   --  --  --  59.5 34.5 33.5 F.sup.1                                                                            F.sup.1             Static Fatigue    F.sup.1                                                                             P.sup.2                                                                             F.sup.1                                                                            F.sup.1                                                                            F.sup.1                                                                            P.sup.2                                                                             P.sup.2                                                                             P.sup.2                                                                             N.sup.3                                                                            F.sup.1             Moduless of Rupture                                                            Ground as ceramic, 1700° F                                                                --   23000                                                                               --  --  --  --   --   --   --  --                  Room T.           19800                                                                               22100                                                                               --  --  --  --   --   --   --  --                  Ground as glass, 1700° F                                                                  --   26200                                                                               --  --  --  --   --   --   --  --                  Room T.           27000                                                                               31000                                                                               --  --  26700                                                                              --   --   --   --  --                  Dimensional Stability, ppm                                                                       -115 -164 -148                                                                               163 -91 --   --   --   --  --                  __________________________________________________________________________      .sup.1 Failed before 200 hours                                                 .sup.2 Passed test N.sup..sup.3 Failed sometimes, passed other times     

                  TABLE II                                                         ______________________________________                                         Composition:                                                                               1       2       6      7     8                                     ______________________________________                                         Component                                                                      SiO.sub.2   77.38   77.38   77.38  77.38 78.79                                 Al.sub.2 O.sub.3                                                                           18.74   19.25   19.10  19.41 18.40                                 Li.sub.2 O  3.88    3.36    3.52   3.21  3.22                                  ______________________________________                                    

While in Table I very few of the compositions were tested for flexural strength precisely according to the test described herein, it is noted all glass-ceramic products of that Table were subjected to the static fatigue test and that those compositions that survived that test at 1700° F obviously had a modulus of rupture of at least 12,000 psi at that temperature.

Comparision of composition 1, having an Li₂ O/Al₂ O₃ ratio of about 0.7, with composition 2, having an Li₂ O/Al₂ O₃ ratio of about 0.6 shows the dramatic difference in the rate of deflection under load and the static fatigue, caused by the difference in this ratio. With respect to composition 9, it is noted that the low amount of ZnO therein did not cause marginal failure in the static fatigue test and that the increase in the rate of permanent deformation or deflection was not as marked as in, for instance, composition 1. On the other hand, the coefficient of thermal expansion was desirably lowered when compared to composition 2. The foregoing facts suggest that one may desire to purposely include the 0.2 weight percent or less ZnO permitted by my disclosure herein when a lower expansion is of special importance in a given application.

In addition to the compositions of the invention which are shown in Table I, further glass compositions of the invention are shown in Table III. The compositions are expressed in weight percent.

                  TABLE III                                                        ______________________________________                                         Composition:                                                                               12      13      14     15    16                                    ______________________________________                                         Component                                                                      SiO.sub.2   76.00   73.55   74.20  73.79 73.64                                 Al.sub.2 O.sub.3                                                                           17.65   19.50   19.25  19.30 19.26                                 Li.sub.2 O  3.00    3.60    3.20   3.56  3.55                                  TiO.sub.2   1.30    1.7     1.45   1.45  1.45                                  ZrO.sub.2   1.75    1.35    1.60   1.60  1.60                                  Sb.sub.2 O.sub.3                                                                           0.30    0.30    0.30   0.30  0.30                                  ZnO         --      --      --     --    0.20                                  ______________________________________                                    

In the following Table IV these compositions are shown on the 100% SiO₂ -Al₂ O₃ -Li₂ O basis. Also shown is the Li₂ O/Al₂ O₃ molar ratio.

                  TABLE IV                                                         ______________________________________                                         Composition:                                                                               12      13      14     15    16                                    ______________________________________                                         Component                                                                      SiO.sub.2   78.63   76.10   76.77  76.35 76.35                                 Al.sub.2 O.sub.3                                                                           18.26   20.18   19.92  19.97 19.97                                 Li.sub.2 O  3.10    3.72    3.31   3.68  3.68                                  Mol Ratio                                                                      Li.sub.2 O/Al.sub.2 O.sub.3                                                                0.580   0.630   0.567  0.629 0.629                                 ______________________________________                                    

When the glasses of the invention are thermally crystallized using the high final crystallization temperatures disclosed herein they form crystals in the keatite solid solution series of the Li₂ O·Al₂ O₃ ·n SiO₂ system where n is greater than 4. The products are crystalline ceramics containing a multitude of substantially homogeneously dispersed, randomly oriented crystals formed by thermal crystallization from the glass, essentially all of which are in their largest lineal dimension less than 5 microns across. While keatite solid solution crystals are present in much greater amounts, either by volume or by weight than any other crystals, other siliceous crystals are sometimes present in small amounts and small amounts of nucleant crystals may also present in the final product. The glasses of the invention can also be heat treated for crystallization at lower final temperatures to obtain glass-ceramics containing high quartz solid solution crystals instead of keatite crystals, but such products do not represent a preferred use of the present glasses.

As will be evident to those skilled in the art, various modifications of this invention can be made or followed in the light of the foregoing disclosure and discussion without departing from the spirit and scope of the disclosure or from the scope of the claims. 

I claim:
 1. A thermally crystallizable glass composition consisting essentially of SiO₂, Al₂ O₃, Li₂ O and the nucleants TiO₂ and ZrO₂, said compositions (1) containing 18.2-20.3 parts Al₂ O₃ per 100 parts by weight of SiO₂ +Al₂ O₃ + Li₂ O (2) having Li₂ O present in a ratio of from 0.55 to 0.63 moles for each mole of Al₂ O₃ present and (3) containing 2.8-5 weight percent (TiO₂ +ZrO₂) based on the total composition, including 1-2 weight percent TiO₂ and 1.2-3 weight percent ZrO₂.
 2. A glass composition of claim 1 containing ZnO in an amount of up to 0.2 weight percent of said glass composition.
 3. A glass composition of claim 1 containing 1.2-1.9 weight percent TiO₂ and 1.4-2 weight percent ZrO₂.
 4. A glass-ceramic resulting from the thermal crystallization of a glass composition consisting essentially of SiO₂, Al₂ O₃, Li₂ O and the nucleants TiO₂ and ZrO₂, said compositions (1) containing 18.2-20.3 parts Al₂ O₃ per 100 parts by weight of SiO₂ +Al₂ O₃ +Li₂ O (2) having Li₂ O present in a ratio of from 0.55 to 0.63 moles for each mole of Al₂ O₃ present and (3) containing 2.8-5 weight percent (TiO₂ +ZrO₂) based on the total composition, including 1-2 weight percent TiO₂ and 1.2-3 weight percent ZrO₂.
 5. A glass-ceramic of claim 4 wherein said glass contains 1.2-1.9 weight percent TiO₂ and 1.4-2 weight percent ZrO₂. 